Tag: Cathleen

Water, Water Everywhere — How Come?

On By rolcottIn Astronomy: Stellar Science, Physics: Quantum Mechanics, UncategorizedLeave a comment

Lunch time, so I elbow my way past Feder and head for the elevator.  He keeps peppering me with questions.

“Was Einstein ever wrong?”

“Sure. His equations pointed the way to black holes but he thought the Universe couldn’t pack that much mass into that small a space.  It could.  There are other cases.”

We’re on the elevator and I punch 2.  “Where you going?  I ain’t done yet.”

“Down to Eddie’s Pizza.  You’re buying.”

“Awright, long as I get my answers.  Next one — if the force pulling an electron toward a nucleus goes as 1/r², when it gets to where r=0 won’t it get stuck there by the infinite force?”

“No, because at very short distances you can’t use that simple force law.  The electron’s quantum wave properties dominate and the charge is a spread-out blur.”

The elevator stops at 7.  Cathleen and a couple of her Astronomy students get on, but Feder just peppers on.  “So I read that everywhere we look in the Solar System there’s water.  How come?”

I look over at Cathleen.  “This is Mr Richard Feder of Fort Lee, NJ.  He’s got questions.  Care to take this one?  He’s buying the pizza.”

“Well, in that case.  It all starts with alpha particles, Mr Feder.”

The elevator door opens on 2, we march into Eddie’s, order and find a table.  “What’s an alpha particle and what’s that got to do with water?”

Alpha particle
Two protons and two neutrons, assembled as an alpha particle

“An alpha particle’s a fragment of nuclear material that contains two protons and two neutrons.  99.999% of all helium atoms have an alpha particle for a nucleus, but alphas are so stable relative to other possible combinations that when heavy atoms get indigestion they usually burp alpha particles.”

“And the water part?”

“That goes back to where our atoms come from — all our atoms, but in particular our hydrogen and oxygen.  Hydrogen’s the simplest atom, just a proton in its nucleus.  That was virtually the only kind of nucleus right after the Big Bang, and it’s still the most common kind.  The first generation of stars got their energy by fusing hydrogen nuclei to make helium.  Even now, that’s true for stars about the size of the Sun or smaller.  More massive stars support hotter processes that can make heavier elements.  Umm, Maria, do you have your class notes from last Tuesday?”

“Yes, Professor.”

“Please show Mr Feder that chart of the most abundant elements in the Universe.  Do you see any patterns in the second and fourth columns, Mr Feder?”

Element Atomic number Mass % *103 Atomic weight Atom % *103
Hydrogen 1 73,900 1 92,351
Helium 2 24,000 4 7,500
Oxygen 8 1,040 16 81
Carbon 6 460 12 48
Neon 10 134 20 8
Iron 26 109 56 2
Nitrogen 7 96 14 <1
Silicon 14 65 32 <1

“Hmm…  I’m gonna skip hydrogen, OK?  All the rest except nitrogen have an even atomic number, and all of ’em except nitrogen the atomic weight is a multiple of four.”

“Bravo, Mr Feder.  You’ve distinguished between two of the primary reaction paths that larger stars use to generate energy.  The alpha ladder starts with carbon-12 and adds one alpha particle after another to go from oxygen-16 on up to iron-56.  The CNO cycle starts with carbon-12 and builds alphas from hydrogens but a slow step in the cycle creates nitrogen-14.”

“Where’s the carbon-12 come from?”

“That’s the third process, triple alpha.  If three alphas with enough kinetic energy meet up within a ridiculously short time interval, you get a carbon-12.  That mostly happens only while a star’s going nova, simultaneously collapsing its interior and spraying most of its hydrogen, helium, carbon and whatever out into space where it can be picked up by neighboring stars.”

“Where’s the water?”

“Part of the whatever is oxygen-16 atoms.  What would a lonely oxygen atom do, floating around out there?  Look at Maria’s table.  Odds are the first couple of atoms it runs across will be hydrogens to link up with.  Presto!  H2O, water in astronomical quantities.  The carbon atoms can make methane, CH4; the nitrogens can make ammonia, NH3; and then photons from Momma star or somewhere can help drive chemical reactions  between those molecules.”

“You’re saying that the water astronomers find on the planets and moons and comets comes from alpha particles inside stars?”

“We’re star dust, Mr Feder.”

~~ Rich Olcott

Planetary Pastry, Third Course

On By rolcottIn Astronomy: Planetary Science, Uncategorized2 Comments

The Al’s Coffee Shop Astronomy gang is still discussing Jupiter’s Great Red Spot.  Cathleen‘s holding court, which is natural because she’s the only for-real Astronomer in the group…  “So here’s what we’ve got.  The rim of the Great Red Spot goes hundreds of miles an hour in the wrong direction compared to hurricanes on Earth.  An Earth hurricane’s eye is calm but the Jupiter Spot’s rim encloses a complex pattern of high winds.  Heat transport and cloud formation on Earth are dominated by water, but Jupiter’s atmospheric dynamic has two active players — water and ammonia.”

“Here’s your pastries, Cathleen.  I brought you a whole selection.  Don’t nobody sneeze on ’em, OK?”

“Oh, they’re perfect, Al.  Thanks.  Let’s start with this bear claw.  We’ll pretend it’s the base of the weather column.  On Earth that’d be mostly ocean, some land surface and some ice.  They’re all rough-ish and steer air currents, which is why there’s a rain shadow inland of coastal mountain ranges.”pastries 2

“Jupiter doesn’t have mountains?”

“We’re virtually certain it doesn’t, Sy.  The planet’s density is so low that it can’t have much heavy material.  It’s essentially an 88,000-mile-wide ball of helium-diluted liquid hydrogen topped by a 30-mile-high weather column.  Anything rocky sank to the core long ago.  The liquid doesn’t even have a real surface.”

<Al and Sy> “Huh?”

“Jovian temps are so low that even at moderate pressures there’s no boundary between gaseous and liquid phases.  Going downward you dive through clear ‘air,’ then progress through an increasingly opalescent haze until you realize you’re swimming.  Physicists just define the ‘surface’ to be the height where the pressure is one atmosphere.  That level’s far enough down that water and ammonia freeze to form overlying cloud layers but hydrogen and helium are still gases.  It could conceivably look like home there except the sky would be weird colors and you don’t see a floor.”

“If the boundary is that blurry, it’s probably pretty much frictionless — weather passes over it without slowing down or losing energy, right?”

“Yup.”

“So there’s way too much slivered almonds and stuff on that bear claw. On this scale it ought to have a mirror finish.”

“Good point.  But now we can start stacking weather onto it.  Here’s my doughnut, to represent the Great Red Spot or any of the other long-lived anticyclones.”

“Auntie who?”

“A-n-t-i-cyclone, Al.  Technical term for a storm that disobeys the Coriolis theory.”

“Uh-HUH. So why’s it do that?”

“Well, at this point we can only go up one level in the cause-and-effect chain.  <pulling out smartphone>  NASA’s Voyager 1 spacecraft sent back data for this this wonderful video

790106-0203_Voyager_58M_to_31M_reduced
Jupiter seen by Voyager 1 probe with blue filter in 1979. One image was taken every Jupiter day (approximately 10 hours).  Credit: NASA

“Basically, the Spot is trapped between two jet streams, one going westward at 135 mph and the other going eastward at 110 mph.  I’ll use these biscotti to represent them.pastries with arrows

“Hey, that’s like a rack-and-pinion gear setup, with two racks and an idler, except the idler gear’s four times as wide as the Earth.”

“A bit less than that these days, Sy.  The Spot’s been shrinking and getting rounder.  Every year since 1980 it’s lost about 300 miles east-west and about 60 miles north-south.  As of 2014 it was about 2.8 Earth-widths across.  And no, we don’t know why.  Theories abound, though.”

“What’s one of them?”

“Believe it or not, climate change.  On Jupiter, not Earth.  One group of scientists at Berkeley tackled a couple of observations

  • Unlike Earth, which is much hotter near the Equator than near the poles, Jupiter’s Equator is only a few degrees warmer than its poles.
  • Three persistent White Ovals near the Great Red Spot merged to form a single White Oval that recently turned red but only around the edges.

Their argument is long, technical and still controversial.  However, their proposal is that merging the three ovals disrupted the primary heat transport mechanism that had been evening out Jupiter’s temperature.  IF that’s true, and if it’s the case that Jupiter’s jet streams are powered by heat transport, then maybe disrupted heat patterns are interfering with  the Great Red Spot’s rack-and-pinion machine.  And maybe more.”

“Big changes ahead for the Big Planet.”

“Maybe.”

~~ Rich Olcott

Planetary Pastry, Second Course

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

We’re still sitting in Al’s coffee shop.  “OK, Cathleen, so Jupiter’s Great Red Spot acts like a hurricane turned inside-out.  Where’s the problem?”

“Just that it goes completely against all the computer models we’ve built to understand and predict hurricane activity.  It’ll take a whole new generation of even more complicated models for Jupiter-like planets.”

“Here’s the doughnuts you asked for, Cathleen.”

“Thanks, Al.  Perfect timing. <drawing on a paper napkin>  Let’s look at hurricanes first, OK, Sy?”

“Sure.”

“We’ll start with this doughnut that I’ve just taken a bite out of.  First thing that happens is that warm ocean water heats up the overlying air.  Warmed air rises, so we’ve got an updraft.”

“And then?”

“The rising air is humid (ocean air, remember?).  As it rises it cools and forces moisture to condense out.  Upward flow stops when the warmed air hits the top of the troposphere.  But there’s still more warm air pushing up the plume.  The cooled air has to go somewhere so it spreads out.  That’s where these red arrows on my paper napkin go horizontal.  The cooled air, loaded with water droplets, is heavy so it starts sinking which is why the red arrows turn downward.  They move back across that ocean water again ’cause they’re caught in the inflow.  Full cycle and that’s number 1 here, got it?”

“Yeah.”

“Hey, Cathleen,  are you gonna need more paper napkins?”Donuts 1
“A couple should be enough, Al, thanks.  Now we get to number 2, the Coriolis thing. That’s always tough to talk students through but let’s try.  The Earth rotates once every 24 hours, right, and its circumference at the Equator is 25,000 miles, so relative to the Sun anything at the Equator is flying eastward at about 1,000 miles per hour.  Any place north of the Equator has to be going slower than that, and further north, even slower.  With me, Sy?”

“Gimme a minute … OK, I suppose.”

“Good.  Now suppose a balloon is floating in the breeze somewhere south of that rising plume.  Relative to the plume, it’ll have eastward momentum.  Now the balloon’s caught in the plume’s inflow but it doesn’t go straight in because of that eastward momentum.  Instead it’s going to arc around the plume.  See how I’ve got it coming in off-center?  Al, would that be clockwise or counterclockwise if you’re looking down from a satellite or something?”

“Umm … counterclockwise, yeah?”

“Mm-hm.  What about a balloon that starts out north of the plume?”

“Uhh … It’ll be going slower than the plume, so the plume gets ahead of it and it’ll arc … hey, counterclockwise again!”

“How ’bout that?  Anywhere in the northern hemisphere, air flowing into a low-pressure region will turn it counterclockwise.  As the inflow draws from greater distances, there’s a greater speed difference to drive the counterclockwise spin.  So that’s number 2 here.  Add those two cycles together and you’ve got number 3, which spirals all around the doughnut.  And there’s your hurricane.”

“Cool.  So how does that model not account for the Great Red Spot?”

“To begin with, the Spot’s in Jupiter’s southern hemisphere so it ought to be going clockwise which it definitely is not.  And there’s no broad band of surrounding clouds — just a lot of structure inside the ring, not outside.  There’s something else going on that swamps Coriolis.”

“So how’s Jupiter different from Earth?  Besides being bigger, of course.”

“Lots of ways, Sy.  You know how labels on healthcare products divide the contents into active ingredients and inert ingredients?  The inert ones just carry or modify the effects of the active ones.  Atmospheres work the same way.  On Earth the inert ingredients are nitrogen and oxygen…”

“Hey, oxygen’s important!”

“Sure, Al, but not when you’re modeling air movement.  The important active ingredient is water — it transports a lot of heat when it evaporates from one place and condenses somewhere else.  The biggest outstanding problem in Earth meteorology is accounting for clouds.”

“You’re gonna tell us that Jupiter’s inactive ingredients are hydrogen and helium, I suppose.”

“Precisely, Sy.  Jupiter has two active ingredients, water and ammonia, plus smaller amounts of sulfur and phosphorus compounds.  Makes for a crazy complicated modeling problem.  I’m going to need more pastries.”

“Comin’ up.”

 

~~ Rich Olcott

Planetary Pastry, First Course

On By rolcottIn Astronomy: Planetary Science, Astronomy: Techniques, UncategorizedLeave a comment

“Morning, Al.  What’s the scone of the day?”

“No scones today, Sy.  Cathleen and one of her Astronomy students used my oven to do a whole batch of these orange-and-apricot Danishes.  Something to do with Jupiter.  Try one.”Great Apricot Spot 1
Cathleen was standing behind me.  “They’re in honor of NASA’s Juno spacecraft.  She just completed a close-up survey of Jupiter’s famous cloud formation, the Great Red Spot.  Whaddaya think?”

“Not bad.  Nice bright color and a good balance of sweetness from the apricot against tartness from the orange.”

“You noticed that, hey?  We had to do a lot of balancing — flavors, colors, the right amount of liquid.  Too juicy and the pastry part comes out gummy, too dry and you break a tooth.  Notice something else?”

“The structure, right?  Like the Spot’s collar around a mushed-up center.”

“Close, but Juno showed us that center’s anything but mushed-up.  <pulls out her smartphone>  Here’s what she sent back.”

GRS 1 @400
Credits: NASA/JPL-Caltech/SwRI/MSSS/Jason Major

“See, it’s swirls within swirls. We tried stirring the filling to look like that but it mostly smoothed out in the baking.”

“Hey, is it true what I heard that the Great Red Spot has been there for 400 years?”

“We think so, Al, but nobody knows for sure.  When Galileo published his telescopic observations of Jupiter in 1610 he didn’t mention a spot.  But that could be because he’d already caught flak from the Church by describing mountains and craters on the supposedly perfect face of the Moon.   Besides, the Jovian moons he saw were much more exciting for the science of the time.  A planet with satellites was a direct contradiction to Aristotle’s Earth-centered Solar System.”

“OK, but what about after Galileo?”

“There are records of a spot between 1665 and 1713 but then no reports of a spot for more than a century.  Maybe it was there and nobody was looking for it, maybe it had disappeared.  But Jupiter’s got one now and it’s been growing and shrinking for the past 185 years.”

“So what is it, what’s it made of and why’s it been there so long?”

“Three questions, one of them easy.”

“Which is easy, Sy?”

“The middle one.  The answer is, no-one knows what it’s made of.  That’s part of Juno‘s mission, to do close-up spectroscopy and help us wheedle what kinds of molecules are in there.  We know that Jupiter’s mostly hydrogen and helium, just like the Sun, but both of those are colorless.  Why some of the planet’s clouds are blue and some are pink — that’s a puzzle, right, Cathleen?”

“Well, we know a little more than that, especially since the Galileo probe dove 100 miles into the clouds in 1995.  The white clouds are colder and made of ammonia ice particles.  The pink clouds are warmer and … ok, we’re still working on that.”

“What about my other two questions, Cathleen?”

“People often call it a hurricane, but that’s a misnomer.  On Earth, a typical hurricane is a broad, complex ring of rainstorms with wind speeds from 75 to 200 mph.  Inside the ring wall people say it’s eerily calm.  The whole thing goes counterclockwise in the northern hemisphere, clockwise in the southern one.”

“So how’s the Great Red Spot different?”

“Size, speed, complexity, even direction.  East-to-west, the Spot is eight times wider than the biggest hurricanes.  Its collar winds run about 350 mph and it rotates counterclockwise even though it’s in Jupiter’s southern hemisphere.  It’s like a hurricane inside-out.”

“It’s not calm inside?”

“Nope, take another look at that Juno image.  There’s at least three very busy bands wrapped around a central structure that looks like it holds three distinct swirls.  That’s the part that’s easiest to understand.” GRS core

“Why so?”

“Geometry.  Adjacent segments of separate swirls have to be moving in the same direction or they’ll cancel each other out.  <scribbles diagram on a paper napkin>  Suppose I’ve got just one inside another one.  If they go in the same direction the faster one speeds up the slower one and they merge.  If they go in opposite directions, one of them disappears.  If there’s more than one inner swirl, there has to be an odd number, see?”

“So if it’s not a hurricane, what is it?”

“Got any donuts, Al?”

~~ Rich Olcott

Twinkle, Twinkle, Tabby’s Star

On By rolcottIn Astronomy: Planetary Science, Astronomy: Stellar Science, Astronomy: Techniques, Uncategorized2 Comments

Al was carrying his coffee pot past our table.  “Refills?  Hey, I heard you guys talking about Tabby’s Star.  Have you seen the latest?”

“Ohmigawd, there’s more?”

“Yeah, Cathleen.  They’ve finally found something that’s periodic.”

“Catch us up, Al.  Cathleen said that the dimmings are irregular.”

“They’ve been, Sy.  But remember Cathleen’s chart that showed big dips in 2011 and 2013, about 750 days apart?  Well, guess what?”

“They’ve seen more dips at 750-day intervals, in 2015 and 2017.”

“Well, not quite.  Nobody was looking in 2015.  But Kickstarter funding let the team buy observing time in 2017.  A dip came in right on schedule.  Here’s the picture. [shows smartphone around]”

WTF 2017 peak after day 5
Visible-light photometry of Tabby’s Star
14-28 May 2017
Image from Dr Boyajian’s blog

Cathleen snorted.  “Damn shame we need crowd-funding to support Science these days.”

“True,” I agreed, “but the good news is that the support is there.  Suddenly you’re scribbling on the back of that envelope.  So what does this chart tell us?”

“I’m sure every astronomer out there will tell you, ‘It’s too soon to say anything for sure.‘  This is raw data, which means it’s hasn’t gone through the usual clean-up process to account for instrumental issues, long-term trending, things like that.  The timing is great, though.  The bottom of this dip is at 18May2017.  The first dip bottomed out 2267 days earlier on 4March2011.  Counting the 2015 case that no-one saw, there’d be three intervals from first to most recent.  2267÷3 makes the average 756 days.  Add 756 to the first date and we’re at 28Mar2013, right in the midst of that year’s complex mess.  It does fit together.”

“So whatever’s causing it has a 756-day orbit?”

“Could be.  I know your next question.  If the eclipsing material were in our Solar System, it’d be a bit outside the 687-day orbit of Mars.  But we’ve already ruled out causes near our solar system.  Tabby’s Star is about 1½ times our Sun’s mass.  That 756-day orbit around Tabby, if it is one, is maybe 30% wider than the orbit of Mars.  But.”

[both] “But?”

“But the dip profiles don’t match up from one cycle to the next.  This dip’s only 2% or so, a tenth of the ones in 2011 and 2013.  Of course, the 2013 event spanned multiple dips so Heaven knows which one we should match to.  Even 2011 and 2017 don’t look the same.  The usual quick-and-dirty way to compare dips is to pair up widths at half depth.  That statistic for 2011 is about a day.  This one is twice that or more.  If the absorber is orbiting the star, it’s changing shape and can’t be a planet.”Tabby in orbit
“So what do we got, Sy?”

“Damifino, Al.  Everything Cathleen just told us points to something like an enormous comet loaded with loose rocks that go flying along random paths away from the star.”

“Sorry, Sy, the infrared data rules out the comet dust that would have to be spewed out along with the rocks.  Besides, someone calculated just how much rocky material would be required to reproduce the dimming we’ve seen already.  You’d need a ‘comet’ somewhere between Earth-size and Jupiter-size, and maybe more than one, and with that much mass the rocks wouldn’t fly apart very well.  Oh, and there’s that long-term fading, which the comet idea doesn’t account for.”

“So we’re down to…”

[sigh] “The explanation of last resort, which astronomers are very reluctant to talk about because journalists tend to go overboard.  Maybe, just maybe, we’re witnessing an advanced civilization at work, constructing a Dyson sphere around a star 1500 light years away.  People have talked about such things for decades.  Think about it — the Sun sends out light in all directions.  Earth intercepts only a billionth of that.  If we could completely surround the Sun with solar panels we’d have access to a billion times more energy than if we covered our own planet with panels.  Better yet, it’s all renewable and producing 24 hours a day.  But even with advanced technology, panels around Tabby’s Star would still radiate in the infrared and we don’t see that.”

My smartphone chirped that same odd ringtone and it was that same odd number, 710-555-1701. “Hello, Ms Baird.”

“The Universe is not only stranger than you imagine, Mr Moire, it’s stranger than you can imagine.”

~~ Rich Olcott

Tabby’s Star — Weird Or Really Weird?

On By rolcottIn Astronomy: Planetary Science, Astronomy: Stellar Science, Astronomy: Techniques, UncategorizedLeave a comment

I needed some time to mull over what Cathleen had told me about Tabby’s Star, so I went to the counter to replenish our coffee and scones. When I returned I said, “OK, let’s recap.  Dr Boyajian’s Planet Hunters citizen scientists found a star that dims oddly.  But I understand there’s lots of variable stars out there.  What’s so special about this one that the SETI project got interested?”

“There’s variable stars and variable stars, but this one shouldn’t vary.  Look, one of the triumphs of 20th-century science is that we pretty much understand how stars work.  You tell me a handful of a star’s properties, things like radius, surface temperature, iron/hydrogen ratio, a couple more, and I can give you its whole life story from light-up to nova.  We’ve catalogued about 70,000 variable stars.  Virtually all of them do episodic brightening — pulsating or flaring up.  There’s about a hundred that dim more or less regularly, but they’re supergiants with cool, sooty atmospheres.  Tabby’s Star is a flat-out normal F-type main sequence star, about 1½ times the Sun’s mass and a little bit warmer.  Like the clean-cut kid next door — no reason to expect trouble from it.”

“So if it’s not the star itself that’s dimming, then something must be getting between it and us.”

“Well, yeah.  The question is what.  There’s so many theories that one pair of authors wrote a 15-page paper just classifying and rating them.”

“Gimme a few.”
Multi-Tabby Star

“Clouds of interstellar dust, for starters.  Sodium’s sparse in stars and the interstellar medium, but it’s got two easily recognized strong absorption lines in the yellow part of the visible spectrum.  Tabby’s sodium lines are broad and weak like you’d expect in a star’s atmosphere, but in the data they’re overlain by sharp, intense absorption peaks that can only come from sodium-bearing gas or dust in the nine-quadrillion-mile journey from there to here.  So there’s dispersed matter in the line of sight, but it can account for at most 35% of the dimming.  Furthermore, an interstellar cloud would have a hard time maintaining structures small enough to produce the sharp dim-and-recover pattern Boyajian found.  Loosely-bound stuff like dust clouds and gas tends to smear out in space.”

“How about comets, or rings, or clumps of asteroids orbiting the star?”

“There’s evidence against all those, but I guess I haven’t mentioned it yet.  You’ve seen the heat lamps over Eddie’s pizza bar?”

“Sure.  Infrared radiation heats things up.”

“And warm things give off infrared radiation.  ‘Warm’ meaning anything above absolute zero.  Better yet, there’s a well-known relation between an object’s temperature and its infrared spectrum.  Rocks or dust anywhere near the star would absorb energy from whatever kind of light and re-radiate it as heat infrared we could see.  The spectrum would show more infrared than you’d expect from the star itself.  And there isn’t any extra infrared.”

“None?”

“Not so’s our technology can detect.  If there’s any there, it’s less than 0.2% of the total coming from the star, nowhere near enough to account for those 8%, 16% and 22% dips.  So no, no comets or rings or asteroid clumps orbiting Tabby’s Star.”

“How about something orbiting our Sun, way far out where we’ve not found it yet?”

“Any light-blocking object near us, like maybe in the Oort Cloud that sends us long-term comets, should produce the same sort of weirdness from Tabby’s near neighbors.  We don’t see that.  One astronomer studied a star only 25 arc-seconds away — steady as a rock.  So whatever’s causing the dimming is probably close to Tabby’s star.  Oh, wait, here’s one more weirdness.  I just saw a report…” [twiddles on tablet] “Yeah, here it is.  Check out this chart.”Dimming montage“You’ll have to unravel that for me.”

“Sure.  The Planet Hunter team was looking for transits, which generally take at most a few days, so the Kepler science team filtered out slow variations before passing the data along.  After Boyajian’s report came out, two Keplerians looked back at the raw data.  I told you about the 3-6% dimming (estimates vary) since 1890.  The raw Kepler data show a 3% drop in four years!”

“I’m starting to think about Dyson Spheres and Larry Niven’s Ringworld.”

“Now you know why SETI got excited.”

~~ Rich Olcott

The Weirdest, And Naughtiest, Star in The Galaxy

On By rolcottIn Astronomy: Planetary Science, Astronomy: Stellar Science, Astronomy: Techniques, UncategorizedLeave a comment

It was an interesting ringtone — aggressive but feminine, with a hint of desperation.  And it was a ringtone I hadn’t programmed into my phone.  The number was intriguing, too — 710-555-1701.  It didn’t add up, so I answered the ring. “Moire here.”

“Hello, Mr Moire, this is Victoria Baird.”

“It’s been a long time, Ms Baird.  What can I do for you?”  Her voice and the memory of her pointed ears sent chills down my spine.

“This time it’s what I can do for you, Mr Moire.  Here’s a tip — Tabby’s star.”  I could hear the italics.  I wanted to ask questions but the line went dead.

Considering the context, I called my Astronomy Department source.  “Morning, Cathleen.  It’s break time, can I buy you some of Al’s coffee and a scone?”

“You’re going to ask me questions, aren’t you?  What am I going to have to bone up on?  I know, it’s Tabby’s Star, right?”

“Got it in one, Cathleen.  Meet you at Al’s?”

“Yeah, give me 15 minutes.”Tabbystar 400

A quarter-hour later we had a table, two mugs of coffee and a plate of scones in front of us.  “So how’d you know I’d be asking about Tabby’s star?  And what is it?  And who is Tabby?”

“Tabby is Tabetha (she spells it with an ‘e’) Boyajian, PhD.  She teaches Astronomy at Louisiana State, does research specializing in high-precision star measurement.  In her spare time she manages a citizen-scientist project called Planet Hunters.  The Hunters get their kicks combing through databases from the Kepler satellite telescope.  They get all excited if the records indicate that a star’s been transited.”

“Oh, like that star-dimming that found the TRAPPIST-1 planets?”

“Exactly.  I think they’ve got over a hundred candidate planetary systems and a couple-dozen confirmed ones to their credit by now.  Anyhow, 2012 was a banner year for them, ’cause they raised an alert on what’s now being called the weirdest star in the galaxy.”

“Which would be Tabby’s Star.  Got it.  But what’s weird about it?”

“Poets like to write about ‘the constant stars.’  This star is world-champion not-constant.  You know how stars seem to flicker when you look at them?”

“Yeah, that’s how I tell them apart from planets.”

“Then you know that the flickering comes from starlight getting messed up going through our turbulent atmosphere.  Astronauts don’t see the flickering.  Neither does Kepler up there, so it can reliably detect miniscule variations in a star’s output.  Virtually all of the 150,000 stars it tracked for four years had rock-steady production.  A few of them occasionally dimmed or flared by maybe a percent, but Tabby’s Star (formally known as KIC 8462852) got the Hunters’ attention when it dimmed by 16%.”

“Twenty times a normal dimming!  Did it stay that way or did the light come back up again?”

“Oh, it came back all right, but the curve on the way up didn’t match the curve on the way down.  That was even weirder.  So the team scoured through the star’s 4-year record and found a dozen events on the 0.05-2% scale, plus one at 8% and another at 21%.”

“21%?  That’s a big shadow.”

“Ya think?  Especially since the between-event timing was seriously irregular and some of those events were complex with three or more separate components.  But that’s not all the weirdness. Those dips lasted for hours or even days, longer than most planetary transits.  After Boyajian and her 48 collaborators published their initial report, which has to have one of the naughtiest titles in the astronomical literature, some other —”

“Wait, a naughty title?  C’mon, don’t tease.”

“OK <sigh>.  The technical term for a star’s light output is flux.  That paper was half about the observations and half about what might be causing the variation.  Assuming the star’s real output is constant, the question becomes, ‘What happened to that missing light?‘  Or as the authors put it, ‘Where’s The Flux?‘  Since then both the paper and the star have been informally referred to as WTF.  OK?”

“OK <sigh>.  So you were saying there’s something else.”

“Yeah.  Some other astronomers went digging in the archives.  WTF has been dimming gradually for at least the past 100 years.  Weird, eh?”

“Yeah.  So what’s causing it?”

“We don’t even have good guesses.”

~~ Rich Olcott

How Many Ways Can You Look at The Sky?

On By rolcottIn Astronomy, Astronomy: Techniques, Mathematics, Uncategorized2 Comments

Cathleen and I were discussing her TRAPPIST-1 seminar in Al’s coffee shop when a familiar voice boomed over the room’s chatter.

“Hey, Cathleen, I got questions.”

“Vinnie?”

“Yeah, Sy, he hangs out with the Astronomy crew sometimes.  You know him, too, huh?”

“From way back.  Long story.”

“What’re your questions, Vinnie?”

“I missed the start of your talk, Cathleen, but why so much hype about this TRAPPIST-1 system?  We’ve already found 3,500 stars with planets, right, and some of them have several.  What’s so special here?”

“You’re right, Vinnie, Kepler-90 has seven planets, just like TRAPPIST-1. (brandishes a paper napkin)  But that star’s more than 60 times further from us than TRAPPIST-1 is.  It’s just too far away for us to be able to learn much more about the planets than their masses and orbital characteristics.  This new system’s only 40 lightyears away, close enough that we’ve got a hope of seeing what’s in the planetary atmospheres.”

(another paper napkin)  “That ties in with the second thing that’s special.  The star’s surface temperature, 2550ºK, is so low that even though its planets orbit very close in, three of them are probably in the Goldilocks Zone.  They’re not too hot and not too cold for liquid water to exist on their surface.  IF there’s liquid water on one of them and IF there’s something living there, we should be able to detect traces of that biochemistry in the planet’s atmosphere.”

Star demographics
Observational data (dots) and four different models
of star count (vertical axis) versus temperature.
Hotter stars are to the left.

(napkin )  “The third special thing is that TRAPPIST-1 is the first-known planet-hosting star in its category — ultra-cool dwarf stars burning below 2700°K.  Finding those stars is hard — they’re small and dim.  No-one really knows how many there are compared to the other categories.  Some models say they should be rare, other models suggest they could be as common as G-type stars like our Sun.  IF there’s lots of ultra-cool dwarfs and IF they generally have planets like G-type stars do, then the category’s a new prime target for exoplanet hunters seeking life-signs.”

“Why’s that?”

“Because it’s easier to spot a small planet around a small star than around a big one.  Transits across TRAPPIST-1 dim its light by 1% or so.  A TRAPPIST-1 planet transiting our Sun would dim it by 1/100th of that.  The same problem hinders planet-finding methods fishing for stars that wobble because a planet’s orbiting around it.”

“Alright, I get that TRAPPIST-1 is special.  My other question is, I heard the part of your talk where you figured the odds on seeing its transits, but you lost me with the word steradian.  My dictionary says that’s an area on a sphere divided by the square of the sphere’s radius. What would that get me?  Where’d your numbers come from?”

“You need one additional piece of information.  If you take any sphere’s total surface area and divide that by r², you’ll always get 4π steradians.  You can use that to convert between absolute surface area and fraction of the sphere.  Mmm…  Sy, you own some land outside of town, yes?”

“A little.”

“And you have mineral rights?”

“Oh, yeah, that’s why I bought it.”

“And they go how far down?”

“All the way to the center of the Earth.”

“So your claim’s actually a pyramid 6370 kilometers deep.  When I moved here I learned it’s impolite to ask how much land someone has.  For round numbers I’ll assume 40 acres, which is about 1,000 square meters.  (tapping keys on her smartphone)  The Earth’s radius is 6.37×106 meters, so Sy’s claim is 1,000/(6.37×106)2 = 2.47×10-11 steradians.  Divide 4π by that and you get … 5.08×1011.  So Earth’s entire surface has room for 5.08×1011 patches matching Sy’s.  Visualize 5.08×1011 pyramids pointing in every direction from Earth’s center.  Now extend each pyramid outward to define a separate patch of sky.  Got that picture, Vinnie?”viewing cones

“Sort of.”

“TRAPPIST-1 is 3.74×1017 meters away.  TRAPPIST-1h’s orbit is a near-circle whose radius is 9.45×109 meters.  It covers π(9.45×109)2/(3.74×1017)2 = 2.00×10-15 steradians on a sphere centered on us. Divide 4π by 2.00×10-15 …  6.27×1015 sky-patches the size of TRAPPIST-1h’s orbit.  They had to pick the right patch to find TRAPPIST-1.”

“Long odds.”

“Yep.”

~~ Rich Olcott

The Luck o’ The (insert nationality here)

On By rolcottIn Astronomy: Stellar Science, Astronomy: Techniques, UncategorizedLeave a comment

“Afternoon, Al.  What’s the ruckus in the back room?”

“Afternoon, Sy.  That’s the Astronomy crew and their weekly post-seminar coffee-and-critique session.  This time, though, they brought their own beer.  You know I don’t have a beer license, just coffee, right?  Could you go over there and tell ’em to keep it covered so I don’t get busted?”

“Sure, Al.  … Afternoon, folks.  What’s all the happy?”

“Hey, Sy, welcome to the party.  Trappist beer, straight from Belgium!”

“Don’t mind if I do, Cathleen, but Al sure would like for you to put that carton under the table.  Makes him nervous.”

“Sure, no problem.”

“Thanks.  I gather your seminar was about the new seven-planet system.  How in the world do the Trappists connect to that story?”

“Patriotism.  The find was announced by a team from Belgium’s University of Liege.  They’ve built a pair of robotic telescopes tailored for seeking out rocks and comets local to our Solar System.  Exoplanets, too.  Astronomers love tying catchy acronyms to their projects.  This group’s proudly Belgian so they called their robots TRAnsiting Planets and Planetesimals Small Telescopes, ergo TRAPPIST, to honor the country’s 14 monasteries.  And their beer.  Mainly the beer, I’ll bet.”

“So the planets are a Belgian discovery?”

“Well, the lead investigator, Michaël Gillon, is at Liege, and so are half-a-dozen of his collaborators.  Their initial funding came from the Belgian government.  But by the time the second paper came out, the one that claimed a full seven planets spanning a new flavor of Goldilocks Zone, they’d pulled in support and telescope time from over a dozen other countries — USA, India, UK, France, Morocco, Saudi Arabia… the list goes on.  So it’s Belgian mostly but not only.”

“I love international science.  Next question — I see the planets are listed as TRAPPIST-1b, TRAPPIST-1c, and so on up to TRAPPIST-1h.  What happened to TRAPPIST-1a?”

“Rules of nomenclature, Sy.  TRAPPIST-1a is the star itself.  Actually, the star already had a formal name, which I just happen to have written down in my seminar notes somewhere … here it is, 2MASS J23062928 – 0502285.  You can see why TRAPPIST-1 is more popular.”

“I’m not even going to ask how that other name unwinds.  So what was the seminar topic this week?”

7 planets
TRAPPIST-1’s planets,
drawn to scale against their star. The
green ones are in the Goldilocks Zone.

“The low probability for us ever noticing those planets blocking the star’s light.”

“I’d think seeing a star winking on and off like it’s sending Morse code would attract attention.”

“That’s not close to what it was doing.  It’s all about the scale.  You know those cartoons that show planets together with their host sun?”

(showing her my smartphone) “Like this one?”

“Yeah.  It’s a lie.”

“How is it lying?”

“It pretends they’re all right next to the star.   7 planets perspectiveThis image is a little better.”  (showing me her phone)  “This artist at least tried to build in some perspective.  Even in this tiny solar system, about 1/500 the radius of ours, the star’s distance to each planet is hundreds to a thousand times the size of the planet.  You just can’t show planets AND their orbits together in a linear diagram.  Now, think about how small these planets are compared to their sun.”

“Aaaa-hah!   When there’s an eclipse, only a small fraction of the light is blocked.”

“That’s part of it.  Each eclipse (we call them transits) dims the measured brightness by only a percent or so.  But it’s worse than that.”

eclipses“How so?”

“All those orbits lie in a single plane.  We can’t see the transits unless our position lines up with that plane.  If we’re as little as 1½° out of the plane, we miss them.  But it’s worse than that.”

“How so?”

“During a transit, each planet casts a conical shadow that defines a patch in TRAPPIST-1’s sky.  You can tile TRAPPIST-1’s sky with about 150,000  patches that size.  There’s one chance in 150,000 of being in the right patch to see that 1% dimming.  In our sky there are over 6×1015 patches the size of TRAPPIST-1h’s orbit.  The team had to inspect the just right patch to find it.”

“With odds like that, no wonder TRAPPIST uses robots.”

“Yep.”

~~ Rich Olcott